Although thallium is a metal belonging to III-A group in the periodic table, it is known that, when it is in a certain ionic form, it takes the similar behaviors in the living body to those of potassium belonging to I-A group. In the normal myocardia, potassium ions are actively incorporated in the myocardial cells by an enzyme in the myocardial cell membranes and thereby are accumulated in the myocardia. This incorporation in the normal myocardia depends mainly upon the local blood flow in the myocardia. When a pharmaceutical agent containing thallium ions which is taking the similar behaviors to those of potassium ions in the body is injected intravenously, the agent becomes distributed to the muscles in the whole body. However, many thallium ions are distributed to the active myocardia but not in the affected parts such as ischemia and the like.
On the other hand, thallium ions are distributed depending upon the blood flow as in the case of cesium ions and as the result thallium ions have the marked tendency to be pooled in the tumor lesions in comparison with other tissues. This accumulation of thallium ions in the tumor lesions is presumed to be caused by substitution of potassium ions with thallium ions present in an enzyme. In addition, the extent of accumulation of thallium ions is largely influenced by the blood flow distribution toward the tumor.
In addition, thallium forms a complex with a certain compound, for example, diethyldithiocarbamate. As the result, the charge of the complex becomes neutral. This fact and high lipophilicity of the complex allow it to easily pass through the blood brain barrier and to be held in the brain over a sufficient period of time.
From the above descriptions, a solution of thallium 201 salt, in particular, thallium[.sup.201 Tl]chloride is useful as a radioactive diagnostic agent which is intravenously injected for examining the cardiac disorders and thyroid gland and lung tumors. And a complex of thallium 201 with a certain compound is useful as a radioactive diagnostic agent which is intravenously injected for diagnosing the brain disorders.
Such the radioactive diagnostic agent containing thallium[.sup.201 Tl]chloride for intravenous administration is prepared generally by diluting thallium[.sup.201 Tl]chloride with a physiological saline to the desired concentration of the radioactivity and filling the resulting solution in a container such as glass vial and syringe vial according to the preparation manner of the injectable solution on the Minimum Requirements for Radiopharmaceuticals in Japan. Thallium[.sup.201 Tl]chloride is prepared by irradiating protons accelerated by the cyclotron to thallium 203 or 205, isolating the generated lead 201 and collecting thallium 201 generated by the radioactive disintegration of lead 201.